The present invention relates to a continuous process for production of polymer by an exothermic polymerization reaction, and to a process for regulating the temperature of a reaction mixture used during this reaction.
Vinyl polymers have been known for a long time, especially styrene-based ones, which are employed in many fields, in particular polystyrenes such as transparent polystyrenes or expandable polystyrenes, styrene copolymers especially with acrylonitrile (SAN), polystyrenes modified with natural or synthetic rubber, such as high impact polystyrenes (HIPS), or ABS.
For economic reasons it is preferable to produce these polymers by continuous processes and at high space-time-yield. Among the continuous processes and according to the field of application of these polymers, there are known processes for polymerization in bulk in the liquid monomer optionally in the presence of a small proportion up to 20% by weight of an inert diluent, or in solution in an inert solvent, or else in aqueous suspension. The polymerization reaction may be of radical type, initiated thermally or with the aid of a catalyst generating free radicals, or of ionic type with the aid of ionic catalyst such as an anionic or cationic catalyst.
In all cases the polymerization reaction of vinyl monomer, for example styrene-based, is a highly exothermic reaction exhibiting a high heat of reaction, for example of 175 kcal/kg in the case of polystyrene. Thus, in this case, a completely adiabatic reaction would result in a self-heating of the reaction mixture by approximately 4.degree. C. for each percentage of conversion of styrene to polymer. It is also known that the high viscosity of the reaction mixture, which increases in step with the progress of the reaction, makes heat dissipation difficult, especially through the walls of the reaction chamber, and tends to produce a polymer of a heterogeneous quality. In particular, any self-heating of the reaction mixture promotes the production of oligomers and of polymers of low molecular weight and the broadening of the molecular mass distribution. A poor control or an insufficient or imprecise control of the polymerization temperature can also have a detrimental effect on the characteristics and the size of the rubber particles formed during the grafting of the high impact-resistant polystyrenes.
Furthermore, it is also known that ionic or radical catalysts are very sensitive to temperature and can decompose prematurely or excessively, resulting, according to the case, in a catalyst deactivation or overactivation.
With the desire of investigating at the same time a process with high space-time-yield, for example higher than 80, preferably more than 100 kg of polymer per hour per cubic metre of reaction mixture, it was noticed that it became extremely important to control with a great fineness the polymerization temperature or the temperature of the reaction mixture during the stages of the process of polymer manufacture. Thus, a refined control of the temperature of the reaction mixture allows the production of a polymer of a better and more uniform quality, for example a polystyrene with a narrower molecular mass distribution or, in the case of high impact polystyrene, a narrower distribution of the particles.
When the polymerization is carried out at a temperature slightly higher than 100.degree. C., for example from 110.degree. to 140.degree. C., the known processes generally employ, as coolant liquid in the cooling systems, water which is cooled by atmospheric cooling, for example with the aid of an atmospheric tower or an air-cooling device. When the desired temperature goes well above 100.degree. C., for example to 150 or 180.degree. C. and when sometimes the temperature exceptionally raises up to approximately 200.degree. C. or even more in the event of heat runaway, the processes may employ, as coolant liquid, an organic heating liquid which does not decompose at these elevated temperatures and which allows the heat of polymerization to be removed efficaciously. The organic heating liquid itself may be cooled by atmospheric cooling, for example with the aid of an air-cooling device. It has also been noted that the difference in temperature between the polymerization reaction mixture and the coldest coolant liquid employed in the cooling system is much greater than 100.degree. C. In these conditions a small fluctuation in the flow rates of the coolant liquids can result in a rapid and insufficiently controlled variation in the temperature of the reaction mixture and consequently in the polymerization temperature. It has been realized that these problems become particularly serious when the exothermic nature of the polymerization is high, for example with a heat of reaction higher than 100 kcal/kg, in particular higher than 150 kcal/kg, and that the polymerization reaction takes place in conditions of a high space-time-yield, for example higher than 80 or more especially than 100 kg of polymer per hour per cubic metre of reaction mixture.
U.S. Pat. No. 4,759,313 discloses an ethylene oxide process whereby residual heat is recovered from a reactor coolant by generating steam. The process comprises removing the heat of the reaction by a boiling coolant in a primary cooling system and by maintaining the coolant in a two phase (vapor/liquid) mixture in the said cooling system. The vaporized coolant is separated in a coolant separator from the liquid coolant which is returned to the reactor. The heat of the reaction is essentially removed by condensing the separated coolant vapor in at least one coolant condenser wherein water circulates and produces steam. The primary cooling system also comprises a coolant surge drum wherein the condensed coolant is separated from non-condensables and is returned to the coolant separator. However, the primary cooling system is a complex device under relatively high pressure comprising a coolant separator, at least one condenser and a coolant surge drum. Steam is only generated from the coolant vapor, and the non-condensable vapor is vented off and lost with detrimental effects to the environment.
Another process is disclosed in French Patent Application No. 2 332 491 for recovering heat in reactions carried out at very high temperatures, e.g. at least 260.degree. C. and generally from 315 to 1090.degree. C., which are not compatible with reactions of vinyl monomer polymerization.
A process has thus been found which makes it possible at the same time to reconcile the economic objectives with the environmental constraints, to improve reaction control, especially by refining the regulation of the temperature of the polymerization reaction mixture and, finally, to produce a polymer of a better and more uniform quality.